I. Field of the Invention
This invention relates generally to very high frequency oscillators for use in radar and communications applications, and more particularly to a millimeter wave microstrip oscillator employing a relatively high power IMPATT diode as its active element.
II. Discussion of the Prior Art
In certain radar communications applications, a need exists for a high frequency carrier oscillator which will develop a power output in the range of 0.5 to 1.5 watts. Other workers have taken advantage of the higher power output available from IMPATT diodes as compared to GUNN diodes but, for the most part, it has necessitated the use of waveguides. There are many applications where size and space constraints will not permit waveguide technology to be employed. Moreover, it is known in the art that high frequency oscillators can be implemented using GUNN diodes in combination with microstrip circuitry. While this combination may solve the space and environmental problems posed by waveguides, they have necessarily employed GUNN diodes as the active element in that the GUNN diode does not present the same problem in matching its internal impedance to the transmission line impedances with which the oscillator is utilized. IMPATT diodes, on the other hand, can provide a substantially greater power output than an oscillator centered on the use of a GUNN diode but, because of the very small negative resistance characteristic of the IMPATT diode, it is difficult to efficiently match that device to a load using microstrip technology.
One of the criteria for oscillation is that the absolute value of the negative resistance of the diode be greater than the transformed load resistance and that the reactance of the diode be the complex conjugate of the transformed load reactance. Because of the small device resistance, it is somewhat difficult, utilizing microstrip technology to provide the necessary impedance matching to affect efficient oscillation.
The Dydyk U.S. Pat. No. 4,494,086 describes a microstrip oscillator in which an IMPATT diode is employed as its active element. The Dydyk circuit arrangement, while it takes advantage of the higher power characteristics of IMPATT diodes, is limited in terms of its ability to provide a variable output frequency. That is to say, the output frequency in the Dydyk circuit is determined by the dimensions of the printed circuit resonator pattern which, for any given circuit, is a fixed parameter.